NOTATIO

weird title, had trouble putting it into words briefly, probably more because I'm not sure of which terminology to use.
anyway, here goes:

two instruments share a staff in and orchestral score.
their material is almost entirely rhythmically the same, so they share stems through most of the passage in question.
there are a few measures where the instruments share SIMILAR rhythmic patterns but not identical (for example, one holds a dotted half, while the other slurs through three quarters).
In those not-quite-identical passages, they split stems, one up, one down.

Their slurs are identical, even in the passages with split stems.

Should only one single slur appear above the top instrument's music where their music is split-stemmed?
Or should they, for those passages, each have their own slur? (taking into consideration that the slurs would still be identical, regardless of stem direction)

I believe that the modern convention is that where there are up and down stemmed voices on a single staff, each part would have its own slur (as well as any other articulation marks), even if the rhythms of the two parts were identical; otherwise there would be no way to show that one part was to be legato and the other was not to be legato. Where the two parts share a stem, the slurs and articulation marks are shown only once, as you said, since it is then assumed that both parts will also have the same articulation.

this is what I thought as well. thank-you for confirming.